The petrology, petrography and geochemistry of anomalous borehole core sequences in the Highveld coalfield, South Africa : a case study for diatreme activity

Abstract:

Three anomalous borehole core sequences from the north eastern Karoo Basin are
examined. The boreholes are located up to 30 km from each other and are
lithostratigraphically completely atypical for the Vryheid Formation, Ecca Group,
Karoo Supergroup. The lithologies of the three boreholes are intensely brecciated for
the most part, while all of the surrounding boreholes reveal normal stratigraphy;
their sedimentary strata are normally horizontal with no faulting present. The only
known disturbances to the Vryheid Formation in the study area are the occurrence
of intrusive mafic dolerite sills and dykes, which are known to have been
contemporaneous with and immediately following the eruption of the Drakensburg
Group basaltic lavas.
The borehole core lithologies are described in detail with reference to their textural,
mineralogical and petrographic characteristics. Mineral and bulk rock chemical data
are presented. Several modes of origin of the brecciated core sequences are
considered, with the primary hypothesis that the brecciation is due to diatreme
activity. A review of diatremes and their mode of emplacement is proposed with
reference to their occurrence within the Karoo Igneous Province, as some diatremes
in the Karoo are associated with dolerite sill emplacement. The isolated occurrences,
lithologies, petrography, alteration and geochemistry of the sequences are used to
argue that the Vryheid Formation, intersected in the form of the three anomalous
boreholes, was disturbed by diatreme activity, which are genetically related to the
late dolerite sill emplacement into the Karoo Supergroup rocks.

Mpumalanga, “the place where the sun rises”, sets the scene for one of the rarest
wetland types in South Africa. They are formed on one of the last remnants of the
ancient African land surface, contain peat and together ...

The coal-bearing Permian Vryheid Formation of the Ecca Group (Karoo Supergroup) was investigated at New Denmark Colliery, situated in the north east section of the Karoo Basin, South Africa. The lithostratigraphy of the ...